Lighter Copter, Less Flight Time

I have a copter I really like… Little 3" prop microcopter. 1402, 8100kv motors. 2s. 190grams.

I built a twin of it, only it’s lighter on a different frame. Also 3". same props and motors. 160 grams.

With the heavier copter I can fly for 7 minutes w/ battery to spare.
With the new lighter copter I can only fly for about 5.5 minutes.

I have two theories. Either this new ESC is much less efficient. Both blheli_32, bot 30 amp. Both have the same settings. Old one is Aikon AK32. New one is Lumenier Micro Razor.

Or, the frame geometry. The old one is kind of H frame, while the new one is X. They both have braces going from the front motor to the rear. I’m no expert in fluid dynamics or anything, but the way I’m imagining things, there would be turbulent air around the arms. The area between the arm and the front to rear brace is pretty tight on the X framed copter… Maybe that entire area is nothing but turbulence and not much air is going down, while the H frame has more space between the arm and the braces. I’ll post a pic of what I’m thinking.
My other reason for this theory is the top speed seems to be lower. But, that is not a scientific observation… Just a few full throttle runs.
Here’s what I’m thinking… Turbulent air in red, blue is the downward air.

I wouldn’t expect either thing to make about 20% difference!!
I do have another Aikon ESC, but really don’t want to rebuild the copter… (ESC is of course at the bottom of the stack… Have to completely disassemble and rebuild to replace it).
Anyone else think my frame/airflow theory might be correct?

This is an interesting topic and opens for good discussion. One of the problems with these small copters is that it is hard to find quality equipment. They are also aerodynamically challenging to get efficient because of the small prop diameter. I agree that the lighter copter should hover longer than the heavier in your case. Comparative tests are best when only hovering.

From looking at your picture, I don’t think the trust differences of the accumulated areas of the arms and braces are large enough to make a significant difference in flight time. The disc area blocked by the arms and braces can be measured or calculated (as what you have done by taking a picture from the top and calculating pixel area of the blocking carbon). Circular and chamfered thinner arms give less drag etc etc. I am not aware of work done in aerodynamic optimization of smaller (even to large) copters, which is readily done with fex CFD-modelling or bench thrust tests. Some designs are dynamically in-efficient. H-configuration should not be more efficient than X in terms of disc area (it is the same length, the prop radius). X- may be easier to get a cog, H sometimes are run with reverse prop direction and it may be harder to get good force distribution etc., but its not related to thrust efficiency.

You probably already thought of these things, but for the sake of discussion, I would focus on the magnitude of lost usable work due to heat dissipation. I think the differences are related to vibration, equipment, as well as how the copters are tuned. The propellers imbalance may be causing vibrations or deviate from cog. A couple of things to look at could be propeller quality, evolving motor and esc temperatures, rpm variations during hoovering. Differentiating noise in the controller is never good, so could loosen up on the d-term and apply filtering (which I am sure you already have done). To increase efficiency of esc’s you could perhaps try to increase pwm frequency (in theory this is important for efficiency), although I am not sure if this can be done with blheli or if the resulting escs are reliable.

Instead of wild guessing, what about to have a log from both copter flying and do a comparison.

I’ve looked for the few things I’ve thought to check… No current sensor in one of them. Otherwise the rpm’s are about the same. Hover throttle is actually higher on the lighter copter.
I’m not sure what else to check. Maybe I’ll hover each of them for a few min and post logs.

I think this is likely the issue…
Everything else should be the same between the two copters. Same motors and props. Same tune. Less vibrations on the new one.
The lumenier ESC gets very hot, just plugged in on the bench… Never had another ESC do the same. It’s burning some power just sitting still.

To eliminate battery issue, use the same battery, fully charged for each of the flying.

To close this out, it was the ESC. I got another Aikon, and can fly for about 2 minutes more!